In
Ham Radio Series number 31, I went into details on the parabolic dish antennas, especially their gain
and radiation beamwidth. Given the tradeoffs in antenna construction, we
always ask if there’s a better way to do things. For example, consider
satellite communications. The satellite TV world generally uses
parabolic dishes; they may not be 12 to 15 foot diameter, circular
dishes like those used in the 1980s (when I was in the design side of that
world), but they’re permanently mounted on a house’s roof, or on a mast in the
backyard or in some other way so that they can always see the geosynchronous
equatorial orbit (GEO). The satellites use that orbit so that the
antennas don’t have to track satellites across the sky – they’re
stationary. Being in the GEO imposes costs on the satellites: they need
more power to get a good signal to a smaller dish from 22,236 miles out in
space. Bigger antennas on the satellites (or the ground) can
ameliorate that. All of that goes into the system design trades and
you’ve got to know the system designers have chosen a combination of antenna
gains on both ends of the link and transmitter power they thought was the best
trade.
Now consider situations like the Starlink internet satellites that aren’t in
GEO. They don’t want the satellites to be that far out because it adds
time for the signals to travel to and from the satellites, which adds latency
to the internet link. Just the limitation of the speed of light, which
is rarely a factor in daily life, adds time. The time for a satellite
signal to reach the Earth’s surface from the GEO is around 1/8 of a second
(119 milliseconds) - double that for two way communications and add any
time added by the hardware and system. Starlink has said that a they are
working towards a latency goal of 20–50 milliseconds and think that 10 msec is
possible. The satellite altitudes seem to only take up around 2 msec due
to the speed of light.
Because the Starlink satellites are much lower in orbit than the GEO, they’re
going to be moving across the sky and the antenna is going to need to track
them. Just based on what we know of the frequencies the Starlink
satellites use and their altitudes, we can’t say much about the required
antenna gain or beamwidth, but with different technology than the parabolic
dish, both may be adjustable.
Consider this side by side comparison between a Dish Network antenna that
would be used to receive TV from GEO to a Starlink antenna and you’ll see a
striking difference. Please let me point out that I have no reason to
believe that these antennas have the same gain, just that they have the same
basic purpose: receiving signals from satellites. While the
Starlink uses several frequency bands, some are the same as the DishHD antenna uses..
The Dish Network antenna is a design that allows the user to put three
different Low Noise Block downconverters (usually just called an LNB), and
each of the LNBs is at a different focal point of the small dish, allowing it
to get signals from three different satellites. Although not very
noticeable in this product photo, the triple feed arrangement on the boom
sticking out from the dish is curved.
The Starlink by contrast, does not have a boom at all and is a sealed
container. I don’t know if this is official in any way, but the antenna
system is referred to as “Dishy McFlatface” and the antenna is flat. It’s not a parabolic reflector and instead
of an LNB sitting above the flat face, there are hundreds of LNBs integrated
into the antenna assembly. The rectangular version of Dishy is newer;
earlier versions were more circular but with portions of the circle cut off,
resulting in a rectangle with rounded corners. Another important
difference is that the Starlink antenna also transmits uplink signals from the
users to the satellites, so it transmits and receives.
So how does one make a flat antenna perform essentially equal to a parabolic
dish? Back in the first article I wrote on gain in antennas, I noted the
basic idea was, “stick more metal in the air,” as long as we all agree that it
can’t be randomly sized and placed chunks of metal. In this case, the
antenna is an array of quarter, half, or one wave long antenna elements fed by
circuits called phase shifters that create delays in the signals to the
radiating elements needed to change the wavefront to the desired shape and
direction. As a result, the antenna elements can combine received
signals in phase creating a much higher amplitude signal or the phase shifters
can create delays between the antenna elements during transmit, creating
focused beams of transmitter power.
Don’t think of phased arrays as something new and exotic you’ve never
seen. All of the commonly used directional antennas are phased arrays; a
Yagi is nothing less than a phased array of half-wave elements which, when
optimized, will be slightly longer (in the reflector elements), or slightly
shorter (director elements) than exactly ½ wavelength. The Yagi is
phased by the mechanical spacing, not by electronic circuitry.
Without getting too deeply into what can be very deep,
a Yagi works by interaction of currents in the elements. Consider a three element Yagi like the one in the first post on
directive antennas (post 28
and the right half of this figure)
The driven element, like any half-wave dipole, radiates mostly perpendicular
to its long axis equally forward and backward; that means into and out of the
screen in this view. When that wave gets to the rear and front elements,
the wave induces a current in them and they re-radiate it with a phase
shift. Some of the re-radiated signal goes back to the driven element,
and some goes the other way. Because of the (nearly) quarter wave
distances between each element, and while this is happening the driven element
continues to advance in its waveform, so that by the time those reflections
get back to the driven element the current on it has advanced almost a half
wave. The timing (phasing) is set up so that signals coming from the
reflector forward add to the next wave at the driven element while the signals
from the director are out of phase when they reach the reflector and
cancel. The phasing of the signals by the physical spacing determines
the forward gain (along with other factors, for sure). It has one driven
element and two parasitic elements, but it's phasing that makes it work.
You might be saying, “OK, but this is just getting some improvement in one
direction, how can a phased array get better in multiple directions?”
The biggest advantage of a phased array antenna, like the ones used in “Dishy
McFlatface,” is that there can be scores of elements, not just three (to 12 or
20, like a Yagi). The second advantage is that they can be arranged in
two dimensional arrays of antennas which can allow the beam of the antenna to
be swept in space, electrically steering the way the antenna is pointed.
A simple phased array can be made from power dividers, programmable
attenuators and phase shifters. Power dividers can be obtained in many
division ratios, but the lowest loss versions tend to be powers of 2.
Passive power dividers tend to be reciprocal; that is they’ll split one input
to two outputs or they’ll combine two inputs into one output, just by
connecting them as you want. That makes a simple eight element
phased array look like this schematically (the circles with the Greek letter
“Sigma” in them are power splitters/combiners – Sigma from the common math
notation of “summing”):
If connected in a line, as this implies, they can point the antenna’s beam
where desired, but only moving in one plane (up or down in this
picture). Connecting another eight elements in a row at right angles to
this one, if fed by twice as much power and another power splitter, the beam
can be pointed in two axes; both up and down and in and out of the
screen. (Duplicate everything to the right of that first power splitter
and cut its input; add a splitter to feed that one with one of its outputs and
use the other output to drive the new schematic page, but you do need a bigger
PA – or a distributed network of smaller PAs).
As shown, the bottom antenna has the least phase shift (delay) and fires
slightly sooner than the one above it, which fires slightly sooner than the
one above that, and so on with the top element delayed the most.
Note that this is just for the transmitter side because most people find it
easier to visualize combining the eight antennas to produce a beam pointed in
different directions, but antennas are reciprocal, too. The same phase
shifts that will transmit in a preferred direction will receive better in that
direction, too. Circuits like this are big business, especially with
Multiple Input/Multiple Output (MIMO) cellular systems like 5G.
Note that they use switched chains of attenuator, phase shifter, Power Amp or
LNA (Low Noise Amplifier), phase shifter, attenuator. Transmit path is
the top four in the dashed oval; receive path is the bottom four. It’s
also possible to approach this by only switching the amplifier between LNA and
PA, using half as many attenuators and phase shifters.
A 5G cellular antenna array based on the Anokiwave AWMF-0139 quad core
IC. (Source)
Unlike the parabolic dish antenna, I can’t provide simple equations for
gain and beamwidth. They’re far too configurable for simple things
like that. They can, however, be designed to provide similar values
for both. In many applications, parabolic dishes have come to be
considered as having too many disadvantages and are being replaced with
phased arrays, even if they are largely mechanical and aren’t electrically
steerable. There are phased arrays of microwave antennas that are
flat panels instead of parabolic dishes used in radar systems, for
example.
